The present invention relates to a fixing apparatus used in an image forming apparatus of a printer, a facsimile, a copier or the like for forming an image by using an electrophotography technology. Particularly, the invention relates to a restricting member for restricting an interval between a surface of a fixing roller and an exfoliating member of the image forming apparatus.
Also, the invention particularly relates to a technology of improving the exfoliating member.
Generally, an image forming apparatus using an electrophotography technology includes a photo conductive member having a photo conductive layer at an outer peripheral face thereof, a charging member for uniformly charging the outer peripheral face of the photo conductive member, an exposing member for forming an electrostatic latent image by selectively exposing the outer peripheral face uniformly charged by the charging member, a developing member for constituting a visible image (toner image) by providing a toner which is a developing agent to the electrostatic latent image formed by the exposing member, a transferring member for transferring the toner image developed by the developing member on a sheet-like record medium such as a sheet and a fixing apparatus for fixing the toner image onto the record medium transferred with the toner image by the transferring member.
The fixing apparatus includes a fixing roller driven to rotate and a pressing roller rotated by being brought into press contact with the fixing roller. The fixing roller has a heating member. The toner is melted to fix on the record medium by heating the record medium while passing the record medium through a press contact portion formed by the fixing roller and a pressing roller.
Further, since a face of the record medium, on which the toner image is fixed, liable to paste on the fixing roller by adhering operation of the toner, there is provided an exfoliating member for exfoliating and guiding the sheet-like record medium passing the press contact portion from a surface thereof.
JP-A-9-134090 discloses a first related fixing apparatus which includes a fixing roller 1 driven to rotate, a pressing roller 2 rotated by being brought into press contact with the fixing roller 1, an exfoliating member (wire member 11) provided to be proximate to the fixing roller 1 for exfoliating and guiding a sheet-like record medium S passing a press contact portion 3 of the fixing roller 1 and the pressing roller 2 from a surface of the fixing roller 1 and restricting members 12 provided at both end portions of the exfoliating member 11 for restricting an interval between a surface of the fixing roller 1 and the exfoliating member 11 by being brought into contact with both end portions of the fixing roller 1 in which outer side portions of the restricting members 12 are arranged on inner sides of end edge portions of the fixing roller 1. Especially, the first related fixing apparatus is disclosed in paragraphs 0011 through 0013, FIGS. 1 through 4 thereof.
According to JP-A-9-134090, although lengths of the fixing roller 1 and the pressing roller 2 are drawn to be the same, when the fixing apparatus is actually constituted, the lengths of the fixing roller 1 and the pressing roller 2 frequently differ from each other.
Further, although the fixing roller is preferably constituted by a gradual inverse crown shape (shape in which a diameter at a central portion thereof in an axis direction is slightly smaller than a diameter at both end portions thereof) in order to prevent a record medium from being wrinkled in the fixing operation, when the fixing roller is formed in the inverse crown shape, the following problem is found to pose.
For example, as shown in FIG. 7, when the fixing roller 1 is constituted by the inverse crown shape and the pressing roller 2 is brought into press contact with the fixing roller 1, an end edge portion 2c of the pressing roller 2 is most strongly brought into press contact with the fixing roller 1. Therefore, an abutment portion of the fixing roller 1 brought into contact with the roller end edge portion 2c of the pressing roller is most significantly worn to bring about a contact mark T1 in a ring-like shape. When a restricting member 12 is brought into contact with a portion of the contact mark T1, the contact state of the fixing roller 1 and the restriction member 12 becomes unstable.
Therefore, it has been found that the interval between the surface of the fixing roller and the exfoliating member is varied, operation of exfoliating the record medium by the exfoliating member is not carried out smoothly and jam (clogging of record medium) is brought about.
By the way, in a case that the fixing roller 11 has a constant diameter in an axis direction thereof and an outer side portion 12a of the restricting member 12 is arranged on an inner side of an end edge portion 1 a of the fixing roller 1 as shown in FIG. 8A, since the restricting member 12 restricts the interval between the exfoliating member 11 and the surface of the fixing roller by being brought into contact with the surface of the fixing roller 1, a contact portion of the surface of the fixing roller is gradually worn by press contact with the restricting member 12 although gradually while the fixing apparatus is operated. The worn portion is produced in a ring-like shape on the surface of the fixing roller as a contact mark. The contact mark is remarkable at portions in correspondence with the both side edge portions of the restricting member 12.
According to the related fixing apparatus shown in FIG. 8A, since an outer side portion 12a of the restricting member 12 is arranged on an inner side of an end edge portion 1a of the fixing roller 1, two streaks of contact marks T1′ and T2′ are produced at portions of the fixing roller 1 in correspondence with both side edge portions 12a and 12b of the restricting member 12.
Also, when the restricting member 12 is brought into contact with the fixing roller 1 having the inverse crown shape as shown in FIG. 8B, it is found that a contact mark T2 is produced at a portion of the fixing roller 1 in correspondence with the outer side portion 12a of the restricting member 12. Because a contact portion of the surface of the fixing roller is gradually worn by press contact with the restricting member 12 although gradually, and the worn portion is produced in a ring-like shape on the surface of the fixing roller as the contact mark T2. When the fixing roller 1 is contracted by a change in temperature of its own as shown in FIG. 8C, the restricting member 12 is slid relative to the fixing roller 1 on an outer side thereof in the axis direction as shown in from an imaginary line to a bold line. The restricting member 12 is brought into contact with a portion of the fixing roller 1 having a diameter larger than that of the case of FIG. 8B and therefore, the interval between the surface of the fixing roller and the exfoliating member is increased by an amount of ΔG which is an amount of a variation of the restricting member 12. As a result, operation of exfoliating the record medium by the exfoliating member is not carried out smoothly and jam (clogging of record medium) is brought about.
Meanwhile, JP-A-11-184300 discloses a second related fixing apparatus which includes a fixing roller driven to rotate, a pressing roller rotated by being brought into press contact with the fixing roller, an exfoliating sheet provided to the fixing roller for exfoliating and guiding a record medium in a sheet-like shape passing a press contact portion of the fixing roller and the pressing roller from a surface of the fixing roller and a support plate for supporting the exfoliating sheet.
Further, the exfoliating sheet has a base member comprised of a heat resistant resin or a metal and a fluororesin layer provided on a surface thereof. The support plate is constituted by a metal. Especially, the second related fixing apparatus is disclosed in paragraphs 0018, 0022 through 0024, FIGS. 1 and 2 thereof.
According to the second related fixing apparatus, the record medium is transported by the press contact portion of the fixing member and the pressing member while being heated and therefore, when the record medium is a medium including moisture of paper or the like, the moisture of the record medium is discharged from an outlet of the press contact portion as steam.
The steam is directed to an upper side, or when the fixing apparatus is provided with blowing device (or sucking device), the steam is directed to a downstream side of an airflow produced by the blowing device.
Therefore, when the exfoliating member is provided on the upper side of the outlet or on the downstream side of the air flow from the outlet of the press contact portion and the exfoliating member is at low temperatures (for example, the exfoliating member is at low temperatures at an initial stage of operating the fixing apparatus and thereafter, the temperature of the exfoliating member is gradually elevated by receiving heat from the fixing member or the like), dropwise condensation is brought about at a surface of the exfoliating member (guide face of the record medium).
The moisture condensed on the surface of the exfoliating member wets the record medium to thereby produce a wet mark on the record medium or bring about a strain of the record medium by the wetting.
Further, in the case of carrying out both faces printing of forming an image on the surface of the record medium and thereafter forming an image also on a rear face thereof, when the rear face side of the record medium is wetted in forming the image on the surface to fix and an image is going to be formed on the wetted rear face side, there is a case in which an electric property of the record medium is changed by wetting the record medium, a toner image cannot be transferred, or melting of the toner is hampered by the wet moisture to thereby bring about a significant defect in the image.